Method for determining the oil content of subterranean formations



1 A946- HENRI-GEORGES 001.1. 2,395,517

METHOD FOR DETERMINING THE OIL CONTENT OF SUBTERRANEAN FORMATIONS Filed March 24, 1942 1 I007, WA TER 0! /002' WA T0? 02 0 Z 0/1. m0! 0; x 0/1. 2

JOURC E ELECTRIC/1L {EA/5R6) INVENTOR Men/'1' Geo/31:: 00)] Patented Feb. 26, 1946 ms'rnonroa DETERMINING THE 011. con- TENT F SUBTERRANEAN FORMATIONS Henri-Georges Doll, Houston, Tex., assignor to Schlumberger Well Surveying Corporation, Houston, Tex, a corporation of Delaware Application March 24, 1942, Serial No. 435,956

4 Claims. (01. 175-182) The present invention relates to a newand improved method for determining the oil content of subterranean formations traversed by a bore hole drilled into the earth.

This application is a continuation-in-part of my copending application for Method and devices for determining the strata traversed by a drill hole," filed February 15. 1938, Serial No. 190,652, now Patent No. 2,317,259, dated April 20, 1943.

Oil is generally found in porous earth formations such as sands, for example. Often salt water is present with the oil in the formations. After the depth and thickness of an oil-bearing formation have been determined by the methods disclosed in prior Patents Nos. 1,819,923 and 1,913,293, for example, tests are usually made for the purpose of determining the relative amounts of oil and water present in it. It is quite important that the data provided by such tests be accurate for they are used in determining whether it would be feasible to exploit the formation commercially, and also in estimating the oil reserve in the formation.

In the present practice. the oil content of an oil-bearing formation is determined by subjecting samples of the formation to chemical analysis. The samples may be obtained in any conventional manner, as. for. example, by means of a sample taker oi the type disclosed in prior Patent No. 2,055,506. While it is possible to obtain much useful information about subterranean formations in this fashion, some difficult; has been experienced in obtaining accurate determinations of the oil content of the formations.

Good petroleum engineering practice requires that the bore hole contain a column of liquid or drilling mud which aids in the drilling operations and serves to prevent the surrounding formations from caving in when the drill stem is removed. At the very considerable depths ordinarily encountered in practice. the pressure of this column of liquid causes it to invade the surrounding permeable formations. sometimes to an appreciable depth. E en if there has been no appreciable invasion of the bore hole liquid into the formations, the cores obtained may be contaminated in passing through the bore hole liquid to the surface of the earth.

It is an object of the present invention. accordingly, to provide a new and improved method for determining, more accurately than has been possible heretofore, the oil content of subterranean formations traversed by a drill hole.

A further object of the invention is to provide a new and improved method of the above character, in which the oil content values obtained are not materially influenced by any invasion of drilling mud into the formations that may have occurred.

Extensive practical experience in oil fields has shown that a particular sand in a given field generally has the same lithological character and contains water of the same salinity throughout the field. However, the relative amounts of oil and'water contained in the sand are generally not the same at all locations in the field. For this reason. it is desirable to determine the oil content of the formation in each well that traverses it.

The invention is based, in part, on the fact that the electrical resistivity of the material constituting a particular formation. uncontaminated by bore hole liquid, is a function solely of the ratio between the quantities of oil and water that it contains. In accordance with the method. the electrical resistivity Of the formation, uncontaminated by the bore hole liquid, is obtained in the bore hole, as described in the above mentioned copending application. and an accurate determination of its oil content is made by comparing the measured resistivity value with the electrical resistivity of prepared samples having the same lithological character and containing different percentages of oil and water of the same salinity.

The invention may be better understood from the following detailed description taken in connection with the accompanying drawing. in which Fig. 1 is a graph showing the relation between the electrical resistivity of different sands and the relative amounts of water and oil contained therein:

Fig. 2 illustrates the porosity and resistivity curves obtained for a particular type of oilbearing formation;

Fig. 3 is a schematic diagram of apparatus for obtaining indications of the electrical resistivity of earth formations traversed by a bore hole; and

Fig. 4 is a graph like that shown in Fig, 1 except that the data are plotted on log-log paper.

For convenience in practicing the method. it is desirable to obtain curves showing the relation between electrical resistivity and oil-water content for each oil-bearing sand in the field under consideration. In order to derive th curve for a sand of any given lit-hological cnaracter, one or more samples of the sand are first obtained by means of a sample taker of the type described in the aforementioned Patent No. 2,055,506, for example. In the laboratory, these samples are first thoroughly cleaned and dried and then impregnated with different percentages of oil and water of the same salinity as the particular sand involved. The electrical resistivity of the sand for these diiferent percentages of oil and water is then measured in any conventional manner, as for example by means of a Wheatstone bridge, and the results are plotted to givethe curve :1 shown in Fig. 1. Similar curves t and c are obtained for other sands in the field having different lithological character and containing water of the same or different salinity.

The salinity of the water contained in any par-,

ticular oil sand in a given ileld may be determined in anydasired manner. Ii oil is already being produced from that sand thromh a producing well is the field, it will generally contain some water and its salinity can be determined in the laboratory in the usual manner. If that cannot be done, then a sample of the sand may b obtained as described above. and the salinity of the water contained therein determined by analysis in the laboratory. As indicated above, such a determination may not be accurate because the water in the sand may be contaminated by the liquid or mud contained in the drill hole. However, in some Instances it will be sumciently accurate for all practical p rposes.

In some cases, it may be possible to determine the salinity of the water contained in a particular sand directly from the resistivity log taken inthewell.asshowninl"ig.2. Thecurved of spontaneous potentials in the drill hole Il indicates the location of permeable formations and may be obtained as described in prior No. 1,913,293. The electrical resistivity curye e may be obtained as described in my above mentioned copending application and it provides indications of the resistivity of the formations in the part not invaded by bore hole liquid. The shapes of the porosity and resistivity loss shown in Uni-8 iisure indicate that there is a permeable sand between the depths A and C which contains oil between the depths A and B and. water between the depths B and C. It is known from experience that the salinity of the water in the oil-bearing sand between the depths A and B is the same as the salinity of the water in the watersand between the depths B and C, which can be determined by reading 01! the electrical resistivity of the water-sand between the depths B and C from the resistivity log e. Knowing the electrical resistivity of the water-sand and the lithoiogical character oi the sand, the salinity of the water may be readily determined.

The salinity of the water contained in a particular sand in a field may also be obtained from a resistivity log taken in a region where the sand is entirely water-bearing. if such a log is available.

Once the curves 0. b, 0, etc., of Fig. l have been obtained for each oil-bearing sand in a given field, they may be used repeatedly to determine the oilcontent at diiferent locations in the hold. For example. the oil content of a formation traversed by a well in the field may be readily determined by measuring, .in situ in the well, the resistivity of the formation in the-part not invaded by bore hole liquid andcomparing it with the curve in Fig. 1 corresponding to the particular formation.

The electrical resistivity of the formations in thepartnotinvadedbyborehoieliquidmaybe' obtainedwithapparatusofthetypeshownin Fig. 8. Similar apparatus for this purpose is described in my above mer'ioned copending application. That figure illustrates what is known in the art as a "two electrode" well 10881 18 circuit, in which two electrodes IO and II are lowered into the bore hole l2. Electrical energy from a suitable source I is supplied to the power electrode II and to a ground point It at the surface of the earth. Meanwhile, indications are obtained, preferably by means of a recording type potential indicating instrument II, of the potential diiierence between the potential electrode ii and a ground point I 8, also located at the surface of the earth.

It is known that the indications provided by the potential indicating instrument I! are a func tion of the electrical resistivity of the formations at a lateral radius of investigation from the bore hole approidmately equal to the spacing of the electrodes II and II. As described in my aforementioned oopen'ding application. this spacing should be so chosen that the electrical resistivity of the formations in the portion not invaded by the bore hole liquid may be obtained. having in mind the thickness of the bed, the diameter of the bore hole, and the diiferenoe between the resistivity of the liquid or mud in the bore hole and the surrounding formations. The proper spacing tobeusedinanygivenfleldmaybereadilyde termined by measuring the electrical resistivity of a given formation with diii'erent electrode spacing: and choosing the spacing which gives the closest value to the true resistivity of the tormetions.

Ifthecurves a, (2,0, etc., showninl'ig.1are plotted on log-log paper, they appear as parallel straight lines which intersect the ordinate axis above the abscissa axis. Experience has shown 0 that these curves always have the same slope and that the position of each curve is a function 0! the lithological character of the formation it represents and of the salinity of the water contained therein. This circumstance greatly facilitates the preparation of the experimentally derived curves. Since the curve 1-0018. 4) is known to be a straight line, it can be'determined by obtaining only two points thereon. Accordingly, the measured resistivity of two sampl s representing twc diiferent oil-water ratios will determine the cum for each sand.

Furthermore, since it is known that the curve: for the diiferent sands are parallel to one another once the curve 1-0 has been drawn. the curve to: any other sand can be drawn if any point thereor is known. Thus, the curve for any other sand may be determined by measuring its resistivits when it contains only water and plotting th4 value obtained as the point h (Fig. 4). The cum h-imaythen bedrawnparallelimthecurvef-q In the manner described above, a plurality 0:

that has been done, the oil content 0! any give: in the field may be determined by measuring its true resistivity in the manner indicatec above and comparing that resistivity with thi curve for the same sand containing water of th: e salinity. The relative percentages of oi in the sand may then be read direct]; of! the curve. For example, reierring to Figun 4. it the curve i represents the formation undo:

In general, the method described above may be practiced advantageously whenever the respective sands in a given field have substantially the same lithological character and contain water of substantially the same salinity throughout the iield. infrequently, sands are encountered which may not have these characteristics. Lenticular sands or small sand lenses, for example. are not of great lateral extent and may be found in only a very. fewwells on 'a particular structure. The majority of oil bearing sands encountered in practice, however, are usually found to have the desired characteristics referred to above.

It will be apparent from the foregoing that the invention enables the oil content of subterranean formations traversed by a drill hole to be determined with greater accuracy than has been possible heretofore. By measuring accurately the resistivity of a formation in the part that is uncontaminated by the bore hole liquid and comparing it with a curve showing the relation between the electrical resistivity of that formation and the relative percentages of oil and water contained therein, the effects produced by invasion of the bore hole liquid into the surrounding formations may be greatly minimized.

While several specific embodiments have been described above, the invention is not intended to be in any way limited thereby, since those embodiments are susceptible of numerous changes in form and detail within the scope of the following claims.

I claim:

1. A method for determining the oil content of oil-bearing formations traversed by a bore hole containing a liquid, comprising taking cores of the formations, determining the lithological characterofthecoresandthesalinityofthe interstitial fluid contained therein, preparing samples having the same lithological character as the cores, impregnating the samples with difrerentproportionsofoilandwaterofthesame salinity ss.the respective formations from which the cores were taken, measuring the electrical resistivity of said samples for said different proportionsofoilandwatenandmeasuringinthe boreholethereaistivitvofportionsoftheformationsthatarenotmaterialb invadedbythe bore hole liquid. and comparing the resistivities of said portions of the formations, as measured inthehorehols,withthesampleresistivities thereto to ascertain the oil oontent of said formations.

aamsthodfosdetsrmining-theoiloontmt 3 ofanoilbearingformationtraversedbyabore hole containing a column of liquid, comprising preparing samples having the same iithological character as the formation and impregnated with diflerent amounts of oil and water of the same salinity as the water in the formation. measuring the electrical resistivities of said samples, measuring in the bore hole the electrical resistivity of a portion of the formation that is not materially invaded by the bore hole liquid, and comparing said measured formation resistivity with the measured resistivities of said samples, whereby the oil content of said formation may be ascertained.

3. A method of determining the oil content of an oil-bearing formation traversed by a bore hole, comprising taking a core of said formation. determining the lithological character of the core and the salinity of the formation water, preparing one or more samples of material having the same iithological character as said formation and containing accurately predetermined proportions of oil and water, said water having the same salinity as said formation water and the number of samples being sufiicient to afford an accurate knowledge of the relation between resistivity and oil-content, water-content ratio over a continuous range of oil-content, watercontent ratio values, determining said relation, measuring in the bore hole the electrical resistivity o' the formation, and comparing said measured resistivity with said relationto determine the oil content corresponding to said measured resistivity.

4. A method of determining the oil content of oil-bearing formations traversed by a bore hole containing a liquid, comprising taking a core of the formation, determining the lithological charactor of the core and the salinity of the interstitial formation water contained therein, preparing a fluid-free, salt-free sample having the same lithological character as'the core, impregnating the sample with a predetermined proportionofoilandwaterofthesamesalinityasthe formation water, measuring the electrical resistivity of said sample for said proportion of oil and water, plotting a curve having a known slope and form through a point having said electrical resistivity and said proportion as coordinates, measuring, in the bore, the electrical resistivity of the formation, and comparing the measured resistivity of the formation with said curve to determine the oil content corresponding to the measured resistivity of the formation.

HENRI-GEORGES DOLL 

